(a) Field of the Invention
This invention relates to a finder optical system suitable for compact cameras, video cameras, electronic cameras or the like.
(b) Description of the Prior Art
Finder optical systems for cameras are available in various types, each of which needs to have a function for indicating frame masks such as focus frames and field frames and to be simple and relatively compact in arrangement, so that an Albada type finder optical system shown in FIG. 1 and a bright frame type finder optical system shown in FIG. 2 have widely been used. Here, in FIG. 1, reference numeral 1 represents an objective lens unit composed of two negative lenses 2, 3 and 4 represents an eyepiece. A semi-transmissive reflecting surface is configured on an image side surface 5 of the negative lens 3 and a reflecting surface 6 for indicating the frame mask is provided on an object side surface of the eyepiece 4. Most of light coming from an object, as indicated by a solid line, is transmitted by the objective lens unit 1 and the eyepiece 4 to reach an observer's eye so that an image of the object is observed, while part of light, as indicated by a broken line, emanates from the objective lens unit 1, then reflected from the reflecting surface 6, and reflected again by the semi-transmissive reflecting surface 5 provided on the negative lens 3 to form an image of the frame mask at the same position as the object image through the eyepiece 4. As a result, the images of the object and the frame mask can simultaneously be observed within the same visual field.
On the other hand, the bright frame type finder optical system depicted in FIG. 2 is provided with a prism 9 having a half mirror 10 between the objective lens unit 1 and the eyepiece 4. Separated from this, a diffusing plate 8a and a light blocking plate 8b having an opening 8c shaped into the frame mask are arranged in parallel with the objective lens unit 1 and a reflecting mirror 7 is provided in the rear thereof. In the finder optical system of the type, light traversing the opening 8c of the light blocking plate 8b, as indicated by a broken line, is introduced into an optical path for observation through the reflecting mirror 7 and the half mirror 10, with the result that the frame mask can be viewed within the same visual field as the object.
Although, of recent years, a zoom finder has come into use even in a finder system through the spread of a camera with a zoom lens, a mechanism for indicating the focus frame and the field frame is the same as that of a conventional Albada type finder optical system or bright frame type finder optical system, except for a real image type finder optical system. For example, optical systems described in Japanese Patent Preliminary Publication No. Sho 61-213817 and Japanese Patent Preliminary Publication No. Sho 63-52114 adopt each the conventional Albada type, and Japanese Patent Preliminary Publication No. Sho 61-160728 makes use of the conventional bright frame type.
Although the merit of the Albada type finder optical system is that the optical system is compact, it has the demerit that ghost and flare are much generated to render a view difficult due to the arrangement of a half mirror with a plane surface normal to an optical axis. On the other hand, while the merit of the bright frame type finder optical system is that the optical system is devoid of the ghost and flare, it has the demerit that provision of an optical path is needed for lighting to render the optical system bulky as a whole. Moreover, it has also another demerit that since a lighting window is arranged in a position independent of the objective lens unit, an unnatural phenomenon is brought about such that the visual field is bright and the indication is not viewed or, to the contrary, the visual field is dark and the indication is brightly viewed, in accordance with the position of the window.